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Newsgroups: comp.lang.ada
Date: Mon, 4 Jan 2021 03:13:52 -0800 (PST)
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Subject: Re: Quick inverse square root
From: AdaMagica <christ-usch.grein@t-online.de>
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Xref: reader02.eternal-september.org comp.lang.ada:61025
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> with INTERFACES; use INTERFACES;
> generic 
> type F is digits <>; 
> type U is mod <>;
> with function SHIFT_RIGHT( n : U; amount : NATURAL ) return U; 
> function G_QUICK_INVERSE_SQRT( a : F ) return F; 
> 
> function G_QUICK_INVERSE_SQRT( a : F ) return F is 

  pragma Assert (F'Base'Size = U'Modulus);

> magic : constant U := U(1.5 * 2.0**(F'Machine_Mantissa - 1) * (F(F'Machine_Emax - 1) - 0.043));
> y : F := a; 
> i : U; 
> for i'Address use y'Address; 
> begin 
> i := magic - shift_right( i, 1 ); 
> return y * (1.5 - (0.5 * a * y * y));
> end G_QUICK_INVERSE_SQRT;